Since 1989, numerous epidemiological studies have reported a significant association between lack of male circumcision (MC) and risk for HIV infection through heterosexual intercourse. These results have led to calls for male circumcision to be considered as an additional HIV prevention strategy. However, there is a consensus among the international health community that a randomized controlled trial (RCT) of MC is needed to control for possible confounding factors, and there are known risks associated with MC that need investigation. The principal objective of this study is to assess the effectiveness of male circumcision in reducing HIV incidence; we will also evaluate complications of the MC procedure, changes in sexual behavior following circumcision, and the biological mechanisms by which the foreskin may increase HIV susceptibility. The study will be conducted in Nyanza Province, Kenya, where the Luo tribe is the main ethnic group, and less than 10 percent of adult men are circumcised. Uncircumcised men aged 18-24 years will be offered voluntary HIV counseling and testing. HIV negative men will be asked to give informed consent for enrolment. All participants will be interviewed to obtain socio-demographic information and to assess behavioral risk factors, and will be examined for medical conditions. Consenting men will be randomly assigned to the treatment (circumcised) arm or the uncircumcised arm. After circumcision men will be checked for complications at several intervals after the procedure. They will be counseled to abstain from sex until healing is complete. Follow-up visits by all 2,000 participants (1,000 circumcised and 1,000 uncircumcised) will occur six-monthly for two years. Uncircumcised men will be offered circumcision at the end of follow-up. All men will be counseled in strategies to reduce their risk for HIV. The primary endpoints will be HIV incidence and surgical complications. Additional outcomes will be STD incidence and behavioral risk. The number of participants will be sufficient to detect a 50 percent reduction in HIV incidence among circumcised men over two years. Additional laboratory studies of foreskin tissue will evaluate the number and density of specialized cells rich in HIV receptors in order to illuminate the biological mechanisms by which presence of foreskin may increase HIV susceptibility.